Gun Jam

I have been making minor adjustments on and off to my QFT 600HR series carb. One issue that I have noticed with that carb (and my holley 670 SA to some dergee) is that (speaking solely of the QFT here) there is a very slight stumble it feels very much like a faint lean stumble in almost any gear below 2,200 RPM. This stumble will go away when even a slight load is applied to the engine but typicality gets worse momentarily before clearing up and can be heard and felt. Air fuel ratios could even reporting back to be smack in the green it could occur at 14.0 to 13.0 but would go away in the upper to mid 12s in most cases vac would be around 17 to 15. as RPM got lower the stumble would get worse unless power was applied.

Tuning this out has always resulted in almost acceptable results OR acceptable results but at the expense up scrweing up idle mixture (very rich) or I believe in some cases Highspeed also rich. If I wanted to get mid range dialed it so I was happy it would cost me Highend or idle.

I typically would make a small change at a time and see if it made things better or worse an air bleed change or main jet or idle mix. It always felt as if the carb had a pretty narrow operating range.

Today I decided to see if timing would improve things I have been running at 13 deg initial. One thing I noticed is after re-adusting Idle mix from pig rich 10.9 afr indicated to compensate for the mid range stumble to where it should be peak vac which happens to be 13.5 inhg @ about 11.9 to 12.4 AFR the car took extra care to come off of a stop or the engine would chatter or bog. RPM had to be boosted higher and clutch had to be let out a tad slower and it would do fine. I thought the chatter might be do to too much initial timing so I backed it down a tad to 11 I think this made no improvement. I then said WTF and tried 16 or 17 initial and took it for a quick drive. I found AFRs to be lean (15s) in low gear low speed cruise power (just off idle ) So i made the idle mix about 1/16th turn richer at each of the 4 corners. This appeared to have solved the lean part throttle cruise issue.

Advancing inital timing from 13 to 17 has appeared to have made a significant improvement in everything during the short drive around the neighborhood (I hope I haven't spoke too soon) The car launches easier and is more forgiving before bogging down during a normal accel, The engine no longer stumbles at part throttle cruise and runs smooth over a much larger AFR reading. It appears that it was running well even during momentary lean situations as high as 15.5 to 16 which in this case occured duing low gear low rpm (2nd gear 1500 20 mph or so) and attempting to roll on slow smooth power. The car also accelerates better power during a fairly hard accle maybe 50% throttle comes on very smooth and with more kick than at 13 initial timing.

It appears im on the right track to eliminate my carb issue and make the carb far easier to tune.

If 17 initial timing is making improvements How much initial timing is safe?

How do I know how far I can go?

At some point its going to make starting the engine not fun.

Do I run a butt load of inital like 20 deg and then re curve my mechanical MSD dizzy to on roll in 14 deg adv for 34 total?

How can I tell what initial timing to not exceed?

Specs as follows
cam: comp hyd roller pn: 35-421-8. intake: 218 exh:224 lsa 110.0
intake: edelbrock air gap rpm
heads: Trick flow twisted wedge 170 intake runner 61cc chamber

Thanks guys!

-Gun

Ps: IT should be noted that I had to remove the intake manifold to replace a collapsed lifter (twice over a few month period) in both cases I found intake valves to have a tar like substance on them it was thick enough to make me wonder what the hell is going on....but I was not in the mood to look into that...now it a good time and its occurring to me that this build up is either due to our crappy gas and is normal or due to retarded timing ...maybe both....???

67mustang302

As much as you can run. On my XFI I'm running 22 at idle iirc, and 31 total. Cranking is like 10-15.

The problem you'll have is that modern distributors are screwed up with initial and total timing setups. Most of them even on the smallest setting add too much timing, so you either have not enough initial or too much total. 20/34 would be good, but it's dictated more but what the total needs to be. MSD have boxes that allow for cranking retard to help starting. Most all factory cars do as well since they typically run 20-30 idle timing (sometimes more than 30).

Also with a 100lsa on that cam, what the O2 tells you at idle (if you use one) is a flat out lie. The O2 will ALWAYS read rich with more valve overlap. So when your gauge says say....14, the chamber is burning fuel at 15 or even 16. If it idles and runs best at 12.5 then that's what it wants. Prolly more like 13.5-14 in the chamber at that point. But the exhaust will always smell and read rich. Just the nature of the cam.

Starfury

You could always hook the vacuum advance to manifold vacuum. Takes a little retuning of the advance curve, but it does give you more advance at idle and cruise.

69mach1377

Your fuel supply may be mucking up your valves. Use only top tier fuel.

Gun Jam

My dizzy is mech adv only it has no vac ability.

I do have a wideband o2 sensor and it reads about 12.0 at idle. I did not set idle mix using the o2 sensor I used my vac gauge and set it to peak vac...right now I am just slightly rich of peak vac nothing to be concerned about. It idles great and does not smell rich....

My real question is: How do I know how much initial I can roll in? I have read that you can get initial timing to match exactly what the engine wants by using RPM. It seems the method is to simply set idle speed low say 650 or 700 simply roll in adv until peak RPM is found. Once adv becomes excessive engine rpm will drop simply retard timing until peak RPM is once again found then you have located a perfect initial timing....IS it really that simple for locating initial timing?

As for total timing:

would I then record my initial, set total to 36 and attempt to get the engine to ping? If no ping try an different advance washer and shoot for say 38 total and check for ping IF yes then retard total timing by 2 deg or so???

If this is true what is the best way to load the engine to determine if total timing is excessive? I have a 5 speed trans could I just throw it in 5th gear at 60 mph and hammer it? Or does the RPM have to be high and under load?


As for setting an initial timing + and adv that doesnt exceed the required total timing what about this

http://www.msdignition.com/Products/...stributor.aspx

My gut instinct says this is a bad idea and will be a pain in the *** because its going to be a tuning addiction plus it looks too complicated to be reliable and prone to failure.

I am going to check to see if anyone makes a precision adv limit washer set for MSD

Thanks

-Gun

TommyK

Lean mixtures such as those found during part throttle operation are harder to ignite and burn slower which is why more spark advance is needed. However I wouldn't go crazy with initial timing with that cam. If it runs good with 17 degrees initial why not leave it there? If you still find issues at tip in maybe the pump cam or shooter need some fiddling with.

Your heads have a very efficient combustion chamber. 34 degrees total timing may be too much. I have read they make best power with 28-30 degrees total timing but I have no personal experience with them.

67mustang302

The best way to find total timing is on a dyno. Barring that, most engines these days with modern heads/cam typically are 28-32 range for total timing. Sometimes even less.

I'd set it based on how it runs best. Give it what it wants.

BuckeyeDemon

four seconds flat makes some limiters that fit the MSD. I have one of them installed and so now i run 20 initial, 32 total.

Gun Jam

So using peak RPM to find initial timing is not a good idea?

I understand the if its okay don't mess with it but thats subjective. I thought it was pretty good before so if I don't try anything else how do I know if its good?

I like the RPM method because its based on an observable fact. It also seems to have yield good results with others. Maybe what Ill do is find out where peak rpm is and see how that compares to where I am now.

Also thanks for the link to the MSD bushings!

thanks guys

-Gun

fastbackford351

I run as much as much as I can. I advance the timing until it starts to ping under WOT then back it off until it doesn't and that gets me damned close.